FIG. 1 shows a typical transmit lineup used in a communications system. A baseband signal is generated [1] which can theoretically be shifted up to a desired carrier frequency and transmitted through the communications medium. However, modern communications systems use amplitude modulation which produces a signal with a large peak to average ratio (PAR). The PAR is also known as the crest factor (CF) of a signal. A signal with a large PAR is difficult to amplify in a real world transmitter because it places stringent requirements on the analog components of the TX path. Specifically, a D/A [2] needs more dynamic range and a power amplifier (PA) [3] becomes very inefficient.
Very often, a crest factor reduction (CFR) module [4] is inserted in the TX lineup. The purpose of the CFR module [4] (also sometimes called a clipping module) is to reduce the PAR of the signal while introducing as few distortions into the signal as possible. Several different CFR algorithms are in existence, but one factor many have in common is that the gain of the CFR module is not constant. For example, it may be that at low TX power levels, the gain of the CFR module is 0 dB. However, at high TX power levels, the gain of the CFR module may be −0.9 dB.
This presents a problem because it is difficult for the baseband signal generation module [1] to guarantee the actual signal power level that will be delivered to the communications medium. The actual TX power level may be lower or even higher than what the baseband signal generator assumes it to be. From a system performance perspective, it would be better if there was a method to guarantee that the gain of the TX path remains constant.
One method that is often used to correct for the gain error of the CFR module is to add a gain module [15] after the CFR module [4], as can be seen in FIG. 2. In the gain module [15], a gain is applied that is equal in value and opposite in sign to the gain going through the CFR module [4]. Thus, the overall gain of the CFR module [4] and the gain module [15] together will be 0 dB.
One of the beneficial features of typical CFR modules is that for a wide range of average input power values, the peak instantaneous value on the output of the CFR module remains more or less constant, or at most changes very slightly. For example, when transmitting at the maximum rated power of the transmitter, the average power on the output of the CFR module may be −13 dB with the peaks located at approximately −5 dB. If the average TX power is reduced to −15 dB, the peaks on the output of a typical CFR module will still be typically located around −5 dB.
The fact that the peak instantaneous power values coming out of the CFR module are constant for a wide range of TX power values is of beneficial to all the components that come after the CFR module. For example, if the peak power values are more or less constant, less headroom is required by the D/A converter [2] since no margin is needed to cover varying peak power levels.
The problem with the prior art described in FIG. 2 is that because the gain of the gain multiplier [15] will be changing, the peak values coming out of the gain multiplier will not be consistent and will vary in time. This places more difficult requirements on the D/A converter [2] and even may place more difficult requirements on a digital predistortion (DPD) system that may be present in the TX lineup.
There is a need for technical solutions for which the peaks of the signal being transmitted is consistent over a wide range of TX power values.